Study On The Modification And Morphology Control Of LiFePO₄ Cathode Materials For Lithium-ion Batteries

With the rapid development of electronic facilities as well as energy and environmental concerning,the applications of lithium-ion batteries have been expanded to high power and high energy products,such as electric vehicles?EVs?and energy storage devices etc.,from the portable electronic products at the beginning of 21st century.Olivine-structured LiFePO₄ is regarded as one of the most promising cathode for lithium-ions batteries due to its low cost,excellent safety and outstanding cycle stability performance.Based on summarizing Li-ion batteries and its cathode materials and analysing the status quote form olivine Li FePO₄ research domestically and abroad,it's known that the deficiency of the material lies in the poor electronic conductivity and sluggish Li⁺diffusion velocity of LiFePO₄.Aiming at these problems,this work focuses on the preparation and modification of LiFePO₄ cathode materials and carries out the following studies:1.The single metal?Mg,Ti,Mn,Co,Ni?doped LiFePO₄ materials are investigated by the first-principles calculation based on density functional theory in this work.In order to investigate the effects of ion doping on the LiFePO₄,a series of LiFe_15/16M_1/16PO₄?M represents the doping element of Mg,Ti,Mn,Co,Ni,respectively?cathode materials are synthesized by solid-state method.Results show that the ions doping can reduce the energy band,resulting in the improvement of electronic conductivity and Li⁺diffusion rate and better electrochemical property.Particularly,the particles of Mg-doped LiFePO₄ show porous structure,which is beneficial for better specific capacity in low discharge rate and cycling performance,delivering a discharge capacity of 166.1 mAh/g at 0.1C.With Ti⁴⁺doping in the lattice,the LiFePO₄ material presents Ti-3d orbit surrounding in fermi surface,resulting in reduced energy band and excellent rate performance.2.On the basis of response surface method?RSM?and central composite design?CCD?,the structure and electrochemical performance of Mg and Ti co-doped Li Fe_1-x-yMg_xTi_yPO₄/C?x=1.32⁴.68,y=1.32⁴.68?materials are prepared for parameters optimization.Consequence of response surfaces show that,with the increasing amount of Mg and Ti ions,the discharge capacity of LiFePO₄ first increases then decreases,indicating the doping of Mg and Ti on Fe site has obviously synergistic effect.Based on the fitted model,the optimal condition is:Mg-doped amount of 2.9%,Ti-doped amount of 3.0%and the sintering temperature of 678.5oC.This optimized LiFe_0.941Mg_0.029Ti_0.030PO₄/C composite exhibits sphere-like particles,resulting in small charge-discharge polarization and enhanced rate performance as well as cycling stability.3.By ultrasonic dispersion method and vacuum sintering method,different kinds of carbon coated LiFePO₄/C composites are synthesized,respectively.And the synthesis factors,such as ultrasonic dispersion time,solvent,surfactant and carbon source are systematically studied.On one hand,results found that,the particle size of raw materials significantly reduces for better carbon coating after ultrasonic treatment.This may be accounting for the broken of particle aggregation during ultrasonic process.With using mixture solvent and polyvinyl-pyrrolidone?PVP?surfactant agent for ultrasonic dispersion 60 min,the obtained LiFePO₄/C shows decreased particles size,well-dispersed and uniform carbon coating.This material exhibits the best electrochemical performance with discharge capacities of 144.8 mAh/g and 60.3 mAh/g at 1C and 10C,respectively.On the other hand,vacuum sintering is found to facilitate the pyrolysis of organic carbon for better graphitization,inhibiting the growth of LiFePO₄ particles.Particularly,the pyrolysis of sucrose turns to porous architecture after vacuum decomposition.Therefore,the sucrose coated LiFePO₄/C composite shows regular spherical grains of about 200 nm,less agglomeration and carbon thickness of 2³nm,demonstrating superior electrochemical performance of 148.6 and 80.1 mAh/g at 1C and 10C rates.4.A series of LiFePO₄ nanoparticles are synthesized via solvothermal method using glycol as solvent,and the influences of Li:P:Fe mole ratio,reaction temperature and reaction time and precursor concentration are systematically investigated.Results show that with using different mole ratio of Li:P:Fe,different types of intermediate products are formed during solvothermal process.Then the as-prepared LiFePO₄ material presents differents morphologies,such as sphere-like,nano-rod and plate-type.Further studies indicate that the shape of LiFePO₄ particles remained as nanoplates at a mole ratio of Li:P:Fe=3:1.5:1,regardless of different reaction temperatures and time.Nevertheless,as the precursor concentration increases from 0.15M to 0.90 M,the morphology of the LiFePO₄ particles changes from spindle-shape to plate-type and then to a hierarchical club-shaped structure.This change of morphology is determined by the adsorption of anions on the?100?plane and the restriction of the attached nuclei in various concentrations during growing and ripening process.Particularly,with the optimal precursor concentration of 0.3 M,the obtained LiFePO₄ square nanoplates show the discharge capacity of 160.7 mAh/g at 0.1 C and excellent high-rate performance?a discharge capacity of 122.5 mAh/g at 50C?at 25oC as well as excellent low temperature property.This Li FePO₄ nanoplates with superior properties can enlarge parictical application for lithum ion batteries.